Olefin-based polymers can be produced with specially designed molecular weight and molecular weight distribution. This process affords some very unique characteristics to the products, and these polymers are finding many applications, for example, as shoe soles in footwear. Currently the materials used in footwear midsole are dominated by ethylene vinyl acetate copolymers (EVA). Olefin-based polymers (for example, POE (polyolefin elastomers) and OBC (olefin block copolymers)) have several advantages, compared with EVA, such as light weight (due to the comparatively low polymer density) which is a general trend in sole design. In addition, the comparatively high melting point of OBC also brings additional advantages such as low shrinkage, low compression set, and improved dynamic fatigue resistance at elevated temperatures, which are also very important in footwear production and use. Currently, OBC has been commercialized in several footwear applications, and it is expected there will be more applications in the future.
The non-polar molecular nature of the above mentioned olefin-based polymers makes them difficult to be bonded, painted, and printed, due to the low surface energy of the polyolefins. For example, in the assembling process, the midsole is bonded to the vulcanized rubber outsole, and to the shoe upper, which is usually made from natural/artificial leather. The existing maximum total olefin-based polymer loading in the midsole formulation cannot exceed 30% (the balance is EVA) due to adhesion issues of higher olefin-based polymer content compositions. Therefore, there is a need to improve the adhesion characteristics of the olefin-based polymer materials, so as to improve bonding, paintability and printability.